Power plant employing extraction steam for steam generation purposes



Feb. 16, 1965 F. J. HANZALEK POWER PLANT EMPLOYING EXTRACTION STEAM FOR STEAM GENERATION PURPOSES Filed June 19, 1962 INVENTOR; FREDERICK J. HANZALEK BY Q'aw ATTORNEY United States Patent 3,169,373 PGWER PLANT EWLOYING EXTRAQ- TIQN STEAM FUR STEAM GENERA- TIGN POSES Frederick J. Hanzalek, Sufiield, Conan, assignor to Qombustion Engineering, Inc., Windsor, Conn, a corporation of Delaware Filed lune 19, 1962., Ser. N 203,549 13 (Ilaims. (Cl. 60-67) This invention relates to steam power plants. More particularly, it relates to an economical operation of a steam power plant operating on a reheat cycle wherein the steam generator incorporates an evaporator, a superheater, and a reheater with which to supply steam to a turbine installation, and wherein a separate secondary steam generator is employed to operate the main steam generator auxiliaries during start-up of the power plant.

In order to prevent corrosion of the pressure parts of the steam generator and turbine which comprise a steam power plant, the steam created by the generator must be maintained within stringent purity limits. To accomplish this, it is necessary to treat the feedwater which enters the steam cycle in order to control the amount of alkalinity, silica, sulphate, chlorides and other potentially deleterious agents carried by the water. This treatment, which normally takes the form of demineralization and chemical addition, represents a substantial portion of the operating costs of a power plant and therefore, in order to approach the most economical operation of such a plant, it is the aim of the power plant designers to employ a minimal amount of water that must be so treated.

At the present time, it is customary in many modern steam turbine power plants to start up the main steam generator by means of a secondary steam generator of smaller capacity which creates sufficient steam to operate the main steam generator auxiliaries, such as the feedwater pump turbine, feedwater heaters, turbine seals, deaerator, and other such equipment, until such time as the main generator unit is capable of creating a sufl'icient amount of steam to sustain operation of this auxiliary equipment. In view of the fact that the steam which operates the auxiliaries is admitted to the main steam cycle wherein a high degree of water purity is required, it is necessary that such secondary steam be free from impurities; therefore, exceptionally pure feedwater must be employed in the creation of this secondary steam.

Soot blowing, on the other hand, is an operation which, while employing steam, does not require a high purity steam since this steam is removed from the cycle, being blown upon the steam generator tubes to loosen and remove the soot that forms thereon and passing out of the generator with the combustion gases. Depending upon the economic dictates of the power plant, steam for soot blowing purposes is presently obtained either from the main steam cycle or from a separately fired steam generator. Both of these methods of performing the operation are expensive; the former because the steam employed is obtained from treated water which is lost to the cycle and the latter, while it may employ low purity steam, because of the expense of tiring a separate generator to create such steam.

it is therefore a principal object of the present invention to achieve a more economical method and means for operating a steam power plant by providing a means wherein low purity steam can be employed to operate the soot bl wing equipment of the main steam generator, such steam being created without the necessity of firing a separate steam generator.

By means of the present invention it is possible to pro vide a high purity steam for operating the auxiliaries of a steam generator unit during start up by means of a conventionally fired secondary generator arrangement 3,159,373 Patented Feb. 16, 1965 which generates such high purity steam from main cycle or treated feedwater while employing the same secondary generator in a nonfiring condition as a closed heat exchanger to create a low purity steam for soot blowing or other low purity steam purposes during normal operation of the main unit; in the latter instance, employing, as a heating medium for the untreated feedwater, the steam exhausted from an extraction stage of the turbine which can be maintained within the main steam cycle.

Briefly, the concept consists of incorporating a tube bundle within the water drum of the secondary geuer ator and passing exhaust steam from a higher pressure stage of the main turbine through the tube bundle in heat exchange relation with low purity feedwater which is admitted to the secondary generator whilesaid generator is in a non-firing condition. The exhaust steam, after giving up its heat, may then be passed to the main condenser and recirculated through the main steam cycle and the low purity steam, so created, admitted to the soot blowers or other low purity steam operated auxiliaries.

By means of the present concept, the high purity steam and condensate is thus not lost to the cycle. During start up of the main steam generator unit, the secondary steam generator is fired in a conventional manner to transform the high purity feedwaterwhich is admitted to the boiler bank of said steam generator into high purity steam to drive the steam operated auxiliaries of the main unit during start up of the latter. Upon reaching its normal operating condition, steam from the main steam generator cycle undertakes the operation of its steam driven auxiliaries and the secondary generator firing is discontinued. Feed to the secondary steam generator is then switched to low purity feedwater which is transformed into steam by means of the heat given up by the steam received from an extraction stage of the turbine. The low purity steam thus developed is then employed to operate the soot blowing apparatus or the like of the main unit.

It is therefore an object of the present invention to provide a method and means for operating a steam power plant in a manner which achieves higher eificiency and economy by imparting thereto the ability to utilizeboth high and low purity grades of steam for those purposes for which each is desirable with said low purity steam being generated by a source of heat which is an integral part of the conventional power plant installation.

It is a further object of the invention to provide a method and means for operating a steam power plant in a manner which permits the use of a low purity grade of steam for the purpose of soot blowing with said steam being generated in the same secondary steam generator that provides lngh purity steam for the main steam generator auxiliaries while said secondary steam generator is in a non-firing condition.

Another object of the invention is to provide a method and means of extracting heat from the exhaust steam of a high pressure stage of a steam turbine installation to be employed in the generation of low purity steam for soot blowing purposes.

A still further object of the invention is to provide a method and means for operating a steam power plant in a manner which permits the secondary steam generator to remain in a heated condition during the operation of the main steam generator to thereby reduce the amount of start-up time required for said secondary unit when it becomes necessary to restart said unit, as, for example, after trip-out of said main steam generator.

Another object of the invention is to provide an economic advantage in the operation of a reheat cycle power plant in which the cost of fuel suitable for firing the secondary steam generator exceeds the cost of fuel suitable for firing in the main steam generator by minimizing the use of said high cost fuel.

Other objects and advantages will become apparent from the following detailed description when read in conjunction with the appended figure which is a diagrammatic showing of the preferred form of the present invention.

With reference to the figure, numeral is a high capacity or main steam generator, 11 illustrates a steam turbine installation operating on a reheat cycle which is directly connected .to the main steam generator 10, and 12 is a steam generator of relatively small capacity which is employed as a secondary unit for the steam power plant. The numeral 13 is shown as representing the high purity steam driven auxiliaries of the main steam generator 10. For the sake ofclarity and simplicity, only a feedwater pump 14, the turbine drive 15 therefor and turbine control valve 15' (are shown as representing the auxiliaries 13; but it is to be understood that such auxiliaries may also include fan drive turbines, a deaerator, feedwater heaters, turbine seals, space heating, and other high purity steam operated devices (not shown), all of which would be connected in line 46 in a manner similar 'to the feedwater pump turbine 15.

The main steam cycle which operates between the main steam generator 10 and the main turbine installation ll employshighpurity feedwater which is transformed into steam in the main steam generator'10 and drives the main steam turbine 11. The fluid which flows within this cycle must be maintained in a condition of high purity in order to prevent harmful corrosivedeposits from forming on the surfaces of theheat exchange tubes or on the turbine internals. Therefore the feedwater which ultimately forms the steam which is cycled through the main steam generator to the turbine must first be subjected to sedimentation or filtration treatment as well'as other costly chemical treatments. a

The main steam generator 10 consists essentially of a combustion chamber 16, an evaporator section 17, a superheater 18, and a reheater 19. Burners generate the combustion gases which transform the high purity feedwater admitted to the main steam generator 10 into steam. a The turbine installationll operates on a reheat cycle and consists'of a high pressure superheat stage 22 anda low pressure reheat stage 23, the latter. stage exhausting into a condenser 24. Under normal operating conditions superheated steam emerges from the superheater section 18 ofthemain steam generator 10 and flows to the high pressure stage 22 of the turbine installation 11 through conduit 25 to operate said stage, such operation being controlled by means of throttling valve 26. After having performed work in the high pressure stage 22 the steam leaves the stage through conduit 29 and is passed through the reheat section 19 of the main steam generator 10 where it is reheated before being admitted'to the low pressure stage 23 of the turbine through conduit 27 and throttling valve 28., The steam is then exhausted from the low pressure stage 23 into a condenser 24, the con; densate being returned to the evaporator section 17 of the main steam, generator 10 by means of the feed pump 14 through line 32. 7

During normal operating conditions of the'main steam generator 10 and turbine installation 11, the main steam generator auxiliaries 13 operate by means of steam which heated steam for operating the main steam generator auxiliaries 13, the steam flowing to said auxiliaries through the conduit 46.. Firing means 38 which is connected to a source of light oil (not shown) is operated by means of a valve 38' and generates the combustion gases which serve as the heating medium to generate the steam produced by the secondary steam generator.

Feedwater is admitted to the secondary steam generator 12.through an inlet line 39 which receives its feedwater supply from two mutually isolated sources, one of which is a source of high purity or highly treated feedwater 40 and the other, a low purity source 41, each of which communicates with the feedwater line 39 by means of a three-way valve 42 which permits flow offeedwaterfrom one source or, the; ot her.;.-,Numeral43 represents a three element flow control system for controlling the feedwater flow to the secondary generator 12 and the water level in drum 34. p 7

According tothe present invention the steam line 54 which communicates with the steam drum 34 of the secondary'steam generator 12 branches ofif into two steam lines one of which, 46, is connected to the main steam generator auxiliaries 13 and the other, 48, communicates with'the soot blowing apparatus 49 positioned within the drum 35are positioned tube bundles 52 and 53 which are serially connected in the steam line 51, the latter forming a branch of the cold reheat line 29 from the high pressure stage 22 of the turbine installation 11 thus permitting a portion of the reheat steam which flows through flows through line 30 into operating relation with said auxiliaries. However, in order to maintain the main steam generator auxiliaries 13 in operationduring start up of the main steam generator 10, a secondary steam generator 12 is provided which supplies operating steam to said auxiliaries. The secondary steam'generator 12 is a unit of relatively small capacity and comprises a combustion chamber 33 and a boiler bank, the latter consisting of a steam drum 34 and water drum 35, said drums being interconnected by means of a plurality of heat exchange tubes 36. Positioned within the combustion chamber 33 is a superheater 37 which provides superline 29 to be passed through the tube bundles 52 and 53 before entering the condenser 24 thereby to give up some of .its heat to the fluid passing through the vessel 47 and drum 35. Temperature control means 55 are provided to regulate the reheat steam flow through line 51. and thereby control'the temperature of the steam passing through line 48.

The operation of the herein-disclosed arrangement is as follows: During start up of the main steam generator 19, burners 38 of the secondary steam generator are oper- 'ated and valve '42 actuated so as to admit high purity feedwater from the source 40 to the auxiliary steam'generator 12 through feedwater line 39. This feedwater is transformed into'steam by means of the gases generated by the burners38 and, valve '45 being opened and valve 50 being closed, the so-created high purity steam passes into the superheater 37 and through line 46 to operate the auxiliaries 13 of the main steam generator unit 10 before being exhausted into the condenser 24 from whence it circulates within the main steam cycle. When the main steam generator 10 and the turbine installation 11 reach their normal operating condition, valve 45 is closed and valve 31 opened so that the high purity steam generated by the main steam generator 10 may undertake operation of the auxiliaries 13. Firing of-the burners 38 of the secondary steam generator 12 is discontinued by the clobine, installation 11 passing through conduit 29 toward the reheater 19 is diverted from said conduit and passes through the tube bundles 53'and 52 by means of line 51 so as to permit, first, a generation of low purity steam in the water drum 35 of the secondary steam generator and, second, a superheating of 'said so-created steam in' the heat exchange drum 47. This superheated low purity steam isthen permitted to flow through line 48 to appa- V ratus49 for soot blowing purposes within the main generator 143 where it is exhausted and removed with the gases of combustion.

From the foregoing it is apparent that the present in vention provides a simple, convenient means for providing a steam power plant with two independent sources of steam of varying degrees of purity, one being sufiiciently pure to be recycled in the main steam cycle and the other being less pure and useful for soot blowing purposes where it can be exhausted to the atmosphere with little expense to the operation of the unit. Also provided by the present invention is a convenient means for providing heat for transforming the low purity feedwater into steam for soot blowing purposes without the necessity of firing a separate steam generator.

Still another important advantage to be derived from the present invention is the fact that the secondary steam generatoris continually maintained in a heated condition when the main generator is operating so that, when it becomes necessary to employ the secondary unit for start up purposes as, for example, after trip out of the main generator, all that is required is to replace the low purity feedwater in the secondary unit with high purity feedwater and ignite the burners, start up of the latter unit being substantially instantaneous since the unit is already up to temperature.

The herein-disclosed arrangement of apparatus is intended to be merely illustrative of the inventive concept and in no way is intended as a limitation of said concept, it being understood that modifications may be devised Without departing from the spirit of the invention recited in the appended claims.

What is claimed is:

l. A steam power plant comprising a main steam generator having both high purity and low purity steam re quirements; a turbine installation operably connected to said main steam generator; a secondary steam generator adapted to produce both high and low purity steam; said secondary steam generator having mutually isolated high purity and low purity feedwater sources of supply; means for creating high purity steam in said secondary steam generator by passing gases of combustion generated therein during start-up of said main steam generator in heat exchange relation with said high purity feedwater; means for passing said high purity steam from said secondary steam generator to satisfy said high purity steam requirements of said main steam generator during startup thereof; means for creating low purity steam during normal operation of said main steam generator including means for passing low purity feedwater admitted to said secondary steam generator in heat exchange relation with high purity steam generated by said main steam generator; and means'for passing said low purity steam to satisfy said low purity steam requirements of said main steam generator.

2. A steam power plant operating on a reheat cycle comprising a main steam generator having high purity and low purity steam requirements; a turbine installation operably connected to said main steam generator; a secondary steam generator having mutually isolated high purity and low purity feedwater sources of supply; means for creating high purity steam in said secondary steam generator by passing gases of combustion generated therein during start-up of said main steam generator in heat exchange relation with said high purity feedwater; means for passing said high purity steam from that created in said secondary steam generator to satisfy said high purity steam requirements during start up of said main steam generator; means for creating low purity steam during normal operation of said main steam generator including means for passing low purity feedwater admitted to said secondary steam generator in heat exchange relation With steam from an extraction stage of said turbine installation, and means for passing said low purity steam to satisfy said low purity steam requirements.

3. In a steam power plant operating on a reheat cycle, the combination of a main steam generator having high purity and low purity steam requirements; a turbine installation operably connected to said main steam generator; a secondary steam generator having mutually isolated high purity and low purity feedwater sources of sup ply; means for passing said high purity feedwater through said secondary steam generator during start-up of said main steam generator thereby transforming said feedwater into steam; means for directing said so-created steam to satisfy said high purity steam requirements of said main steam generator; means for replacing the flow of high purity steam to satisfy said high purity steam requirements from said secondary steam generator to said main steam generator upon the latter achieving normal operation; means for replacing the flow of high purity feedwater through said secondary steam generator with low purity feedwater; means for effecting a transfer of heat from the high purity steam generated by said main steam generator to said low purity feedwater thereby transforming said feedwater into steam; and means for passing said so-created low purity steam to said main steam generator for satisfying the low purity feedwater requirements thereof.

4. In a steam power plant operating on a reheat cycle, the combination of a main steam generator having high purity steam operated auxiliaries and low purity steam operated auxiliaries; a turbine installation operably connected to said main steam generator; a secondary steam generator having firing means which are operated during start-up of said main steam generator; mutually isolated low purity and high purity feedwater sources of supply connected to said secondary steam generator; means for flowing high purity feedwater through said secondary steam generator during start-up of said main steam generator whereby said feedwater may be transformed into steam; means for passing said so-created high purity steam to said main steam generator auxiliaries; means rendering said secondary steam generator firing means inoperative during normal operation of said main steam generator; means for operating said high purity steam operated auxiliaries by steam created in said main steam generator during normal operation thereof; means for replacing said high purity feedwater flow in said secondary steam generator with low purity feedwater; means for effecting a transfer of heat from the high purity steam from an extraction stage of said turbine to said low purity feedwater thereby transforming said feedwater into steam; and means for passing said so-created low purity steam to said low purity steam operated auxiliaries.

5. In a steam power plant operating on a reheat cycle, the combination of a main steam generator having auxiliaries operated by high purity steam and auxiliaries operated by low purity steam; a turbine installation operably connected to said main steam generator; a secondary steam generator including firing means and steam generating tubes; mutually isolated high purity and low purity feedwater supply means communicating with said steam generating tubes; means operating said secondary steam generator firing means during start-up of said main steam generator; means for flowing high purity feedwater through said steam generator tubes during start-up of said main steam generator whereby steam is created; means for delivering said so-created high purity steam to said high purity steam operated auxiliaries; means rendering said secondary steam generator firing means inoperative during normal operation of said main steam generator; means for operatingsaid high purity steam operated auxiliaries by steam created in said main steam generator during normal operation thereof; means for replacing said high purity feedwater flow in said steam generating tubes with low purity feedwater; means for effecting a transfer of heat from the steam from an extraction stage of said turbine to said low purity feedwater in said secondary steam generator thereby;'trans forming said low purity feedwater into steam; and means for passing said so-created low purity steamfto said low purity steam operated auxiliaries.

6. In the combination of a turbine installation operating on a reheat cycle, a main steam generator including a reheat stage operatively connected to said turbine, said main steam generator having auxiliaries operated by high purity steam and auxiliaries operated by low purity steam; a secondarysteam generator having steam connections to said auxiliaries, said secondary steam generator including firing means'operative during start-up of said main steam generator and steam generating means capable of generating both high purity and low purity steam; the improvement comprising mutually isolated high purity and low purity feedwater supply means communicating with said steam generating means; means for supplying high purity feedwater to said steam generating means during start-up of said main steam generator thereby transforming said high purity feedwater into steam; means for directing said so-created steam to said high purity steam operated auxiliaries; means for replacing said high purity steam flow from said secondary steam generator to said auxiliaries with hi h urit steam from said main steam enerator l a during normal operation thereof; means rendering said secondary steam generator firing means inoperative; means steam and auxiliaries operated by low purity steam; a

secondary steam generator having steam connections to said auxiliaries, said secondary steam generator comprising firing means operative during start-up of said main steam generator and a boiler bank including a steam drum, a Water drum, and a plurality of heat exchanger tubes interconnectingthe two; mutually isolated high purity and low purity feedwater supply means communicating with said boiler bank; means for supplying high purity feedwater to said boiler bank during start-up ofv said main steam generator thereby transforming said feedwater into steam by passing the same in heat exchangerelation with combustion gases generated by said firing means; means for directing said so-created steam to said high purity steam operated auxiliaries; means for replacing said low purity steam flow from said secondary steam generator to said auxiliaries with high purity steam from said main steam generator during normal operation thereof; means renderingsaid secondary steam generator firing means inoperative; means replacing the high purity feedwater supply to said boiler bank with low purity feedwater; heat exchanger means including a tube bundle positioned in heat exchange relation with feedwater in said boiler bank, said tube bundle receiving exhaust steam from a high pressure stage of said turbine thereby transforming said low purity feedwater into steam; and means for directing said so-created low purity steam to said low purity steam operated auxiliaries. I

8. The combination of a turbine installation operating on a reheat cycle; a main steam generator including a reheat stage operatively connected to said turbine, said main steam generator having auxiliaries operated by high purity steam and auxiliariesoperated by low purity steam; a secondary steam geneartor having steam connections to said auxiliaries; said secondary steam genera-tor comprising firing means operative during start-up'of said main steamgenerator and a boiler bank including a steam drum, a water drum and a plurality of heat exchanger 8 tubes connecting the, two; mutually isolated high purity and low purity feedwater supply means communicating with'said boiler bank; means for supplying high purity feedwater to said'boiler bank during start-up of said main steam generator thereby transforming said feedwater into steam by passing the same in heat exchange relation with combustion gases generated by said firing means; means for directing said so-create'd steam to said high purity steam operated auxiliaries; means for replacing said high purity steam flow from said auxiliary steam generator to said auxiliaries With high purity steam flow from said main steam generator during normal operation thereof; means rendering said secondary, steam generator firing means inoperative; means for replacing the high purity feedwater supply to said boiler bank with low' purity feedwater; heat exchange means comprising a tube bundle located in said water drum; a vessel in fluid communica tion with said boiler bank; aftube bundle located in said vessel; conduit means fluidly connecting-the exhaust port of a high pressure'st-age' of said turbine to the-reheat stage of said main steam generator, said conduit means including means connecting said tube bundles in series relation; means connecting said boiler bank and said vessel in fluid circulation'thereby providing means to generate superheated low purity steam for said low purity steam'operated auxiliaries; and means .for directing said so-created superheated, low purity steam to said low purity, steam operated auxiliaries.

9. A method'of supplying low purity steam to satisfy the low purity steam requirements of a steam power plant including a main steam generator, a turbine installation operating on a high purity steam cycle and a secondary steam generator operable during start-up of' said main steam generator to operate the auxiliaries thereof comprising the steps of introducing low purity feedwater to said secondary steam generator when the same is in a non- 'firmg condition during normal operation of said main steam generator and turbine installation, extracting high purity steam from said high purity steam cycle, passing said high purity steam in heat exchange relation with said low purity feedwater in said secondary steam generator thereby transforming said feedwater into steam, and passing said so-created low puritysteam to satisfy the low pur- 1ty steam requirements of said steam power plant.

10. A method of employing a secondary steam generator for supplying high purity steam to the high purity steam operated auxiliaries of a main steam generator operably connected to a turbine installation during start-up of said main steam generator and for supplying low purity steam to the low purity steam operated auxiliaries operably related to said mainsteam generator during normal operation thereof comprising the steps of: generating steam in said secondary steam generator 'by passing high purity feedwater in heat exchange relation with combustion gases. developed in said secondary steam generator, employing said high purity steam to operate said high purity steam operated auxiliaries of the main steam generator during start-up thereof, employing high purity steam generated in said main steam generator to. operate said high purity steam operated auxiliaries during normal operation of said main steam generator, discontinuing the generation of high purity steam in said secondary steam generator and generating low purity steam therein by passing low purity feedwater in heat exchange relation with high purity steam generated by said main steam generator, and employing said low purity steam to operate said low purity steam operated auxiliaries during normal operation of said main steam generator.

11. A method of employing a secondary steam generator for supplying high purity steam tothe steam oper ated auxiliaries of a main steam generator operably connected to a reheat-type turbine installation during start-up of said main steam generator and for supplying low purity steam to the low purity steam operated auxiliaries operably related tosaid main steam generator during normal operation thereof comprising the steps of: admitting high purity feedwater to said secondary steam generator while firing the latter to transform said feedwater into steam; passing said high purity steam to the high purity steam driven auxiliaries of said main steam generator until such time as said main steam generator can create suificient steam to operate said auxiliaries; discontinuing the firing of said secondary steam generator and the high purity feedwater flow thereto; introducing low purity feedwater to said secondary steam generator in heat exchange relation with exhaust steam from a high pressure stage of said turbine installation thereby transforming said W purity feedwater into steam, and passing said low purity steam to the low purity steam operated auxiliaries of said main steam generator.

12. A steam power plant operating on a reheat cycle comprising a main steam generator having both high purity and low purity steam requirements and a turbine installation operatively connected thereto; a secondary steam generator capable of generating high purity steam operatively connected to said main steam generator for satisfying the high purity steam requirements thereof during start-up of said main steam generator, means for passing low purity feedwater in heat exchange relation with extraction steam from said turbine during normal operation of said main steam generator for satisfying said low purity steam requirements thereof.

13. The combination of a turbine installation operating on a reheat cycle; a main steam generator including a reheat stage operatively connected to said turbine, said main steam generator having auxiliaries operated by high purity steam and auxiliaries operated by low purity steam; a secondary steam generator having steam connections to said auxiliaries, said secondary steam generator comprising firing means operative during start-up of said main steam generator and a boiler bank including a steam drum, a

Water drum, and a plurality of heat exchanger tubes connecting the two; mutually isolated high purity and low purity feedwater supply means communicating with said boiler bank; means for supplying high purity feedwater to said boiler bank during start-up of said main steam generator to transform said feedwater into steam by passing the same in heat exchange relation with combustion gases generated by said firing means; means for directing said so-created steam to said high purity steam operated aux iliaries; means for replacing said high purity steam flow from said secondary steam generator to said auxiliaries With high purity steam flow from said main steam generator during normal operation thereof; means rendering said secondary steam generator firing means inoperative; means for replacing the high purity feedwater supply to said boiler bank with low purity feedwater; heat exchange means including a tube bundle located in said Water drum; conduit means fluidly connecting the exhaust port of a high pressure stage of said turbine to the reheating stage of said main steam generator, said conduit means including means communicating with said tube bundle to generate low purity steam for said low purity steam operated auxiliaries; and means for directing said so-created low purity steam to said low purity steam operated auxiliaries.

References Iited in the file of this patent UNETED STATES PATENTS 1,653,560 Gleichmann Dec. 20, 1927 1,781,368 Davidson Nov. 11, 1930 1,838,G07 Smith Dec. 22, 1931 2,193,863 Eglofi Mar. 19, 1940 2,346,179 Meyer et al Apr. 11, 1944 FOREIGN PATENTS 666,255 Germany Oct. 14, 1938 

12. A STEAM POWER PLANT OPERATING ON A REHEAT CYCLE COMPRISING A MAIN STEAM GENERATOR HAVING BOTH HIGH PURITY AND LOW PURITY STEAM REQUIREMENTS AND A TURBINE INSTALLATION OPERATIVELY CONNECTED THERETO; A SECONDARY STEAM GENERATOR CAPABLE OF GENERATING HIGH PURITY STEAM OPERATIVELY CONNECTED TO SAID MAIN STEAM GENERATOR FOR SATISFYING THE HIGH PURITY STEAM REQUIREMENTS THEREOF DURING START-UP OF SAID MAIN STEAM GENERATOR, MEANS FOR PASSING LOW PURITY FEEDWATER IN HEAT EXCHANGE RELATION WITH EXTRACTION STEAM FROM SAID TURBINE DURING NORMAL OPERATION OF SAID MAIN STEAM GENERATOR FOR SATISFYING SAID LOW PURITY STEAM REQUIREMENTS THEREOF. 